This invention relates to a circuit breaker provided with an arc shield which covers a movable contact arm and its contact-pressure spring.
FIGS. 1 and 2 show a conventional circuit breaker as disclosed in Japanese Kokai No. 57-79823, wherein a case or housing 1 comprises a cover 1a and a base 1b. A stationary contact arm 2 carries a stationary contact 2a and is fixed to base 1b. A frame 3 is mounted to the base by screw 4. An operating handle 5 rotates about a fulcrum point 3a of the frame. A cradle 6 is rotatably coupled to the operating handle. An upper link pin 7 is provided on the cradle. An over dead-center toggle or link mechanism 8 comprises an upper link 8a and a lower link 8b interlocked by a connecting pin 9. A helical tension spring 10 urges the upper link 8a against pin 7 by biasing the connecting pin 9 upwardly relative to the operating handle. A contact mounting arm 11 is rotatably mounted to the base 1b and supports a movable contact arm 12 for rotation about a shaft 13. The movable contact arm carries a movable contact 12a. A contact-pressure spring(s) 14 on shaft 13 biases the movable contact arm in a clockwise direction, as best seen in FIG. 2. A fixed conductor 15 is mounted to the base 1b, and a flexible conductor 16 is connected between the fixed conductor 15 and the movable contact arm 12. An electromagnetic device 17 having a movable iron core 17a is mounted to the base 1b. A bimetal 18 has an adjusting screw 18a. The core 17a and the adjusting screw both engage a trip bar 19, as does a latch 20. A latch 21 is disposed between the cradle 6 and the latch 20. The distal end of the lower link 8b is pivoted about the movable contact arm shaft 13.
In operation, when the circuit breaker is closed (not shown in the figures) current flows through the stationary contact arm 2, the engaged stationary and movable contacts 2a, 12a, the movable contact arm 12, the flexible conductor 16, and the fixed conductor 15. When the handle 15 is rotated counterclockwise to open the breaker, the connecting pin 9 of the link mechanism 8 is pulled past the dead center point and up by the helical tension spring 10, and the movable contact arm 12 is attendantly pulled up together with mounting arm 11 to separate the movable and stationary contacts. When a small but continuous overcurrent flows the bimetal 18 bends to rotate the trip bar 19 and trip the breaker in a known manner. When a sharp overcurrent spike flows the movable iron core 17a is actuated to rotate the trip bar and open the breaker in a similar manner.
When such a conventional circuit breaker is tripped, particularly in response to a short circuit or an overload, the arc generated tends to flash over to and along the movable contact arm and thus impair the performance characteristics of the breaker, and to weld the turns of the spring 14 together and/or to the components engaged thereby.